The visual field is that physical space, measured in degrees eccentric to fixation, in which the human eye can perceive images and detect motion. Many causes, ranging from trauma, infection, cancer, autoimmune diseases, toxins, degeneration, or genetic etiology, can alter or disturb the visual field, often in specific, characteristic ways. Diagnosis of the precise, underlying cause of the loss of visual field and characterization of the nature and extent of the loss is essential for prognosis of, and analysis of treatments for, these conditions.
Kinetic visual field testing (perimetry) of the full visual field is commonly used to determine the visual field of subjects with retinitis pigmentosa (RP). Kinetic perimetry utilizes a moving test target, and is time-consuming, dependent upon the availability of a skilled perimetrist, and difficult to quantify, which limits its usefulness in monitoring disease progression and treatment efficacy. By contrast, full-threshold static perimetry, in which retinal sensitivity is measured at multiple stationary test sites in the visual field, is much more objective, can be performed by a wider array of technical personnel, and produces digital data that are suitable for statistical analysis. However, whereas this approach has the advantage of reducing the data to a single variable (the mean deviation [MD], which is a quantitative variable designating overall retinal sensitivity), all topographic information about the location where the visual field changes exist or are changing with time, is lost.
The progression of RP within the retina is far from uniform, and midperipheral regions are the most likely to sustain the earliest and most severe damage. The disease, in most cases, progresses in a symmetrical fashion when examined along temporal-to-nasal and superior-to-inferior axes; thus, any deviation from this expected natural history is lost using a single parameter such as the mean deviation. Special, fast static perimetry programs exist for glaucoma (e.g., the SITA and other Bayesian probability-based threshold determination algorithms and test strategies), but the spatial characteristics and progression of field defects in RP are very different from those of glaucoma. Conventional, full-threshold static perimetry algorithms can assess visual fields in subjects with RP, but the time involved to test each eye is often double that necessary for the more rapid SITA algorithms designed for glaucoma. This increased testing time adds to subject fatigue and decreases the reliability of the data. Test-retest variability is a major limitation in visual field testing, and as a result, limits the usefulness of SITA for monitoring RP.